The objective of this program is to test a unifying hypothesis for the action of fat-soluble vitamins as regulators of protein synthesis in higher mammals. Studies currently in progress deal with the metabolism and actions of vitamins K and E. Prothrombins from rat and chick plasma has been purified, their molecular weights, and amino acid composition have been determined. Their N-terminal amino acids and finger prints are being determined. They are also being used as antigents to develop antisera as a reagent for detection of small amounts of these prothrombins in isolated studies of protein synthesis in vitro. Cell-free systems have been constructed for the biosynthesis of prothrombin. The regulatory protein which binds vitamin K and the coumarin drugs at allosteric sites is being sought by studies of binding radioactive coumarin and vitamin K1. On the basis of kinetic evidence a model has been proposed which involves the association of a regulatory protein binding vitamin K1 and coumarin to the 60-S particle of the liver ribosome. It is believed that this protein serves as an initiation factor for prothrombin biosynthesis in the mammal, and experiments are in progress to validate this hypothesis.